What Would A Fan Do Inside a Sealed Flashlight?

Hello Guys,

Just throwing around the possible idea of what a fan might do in a sealed flashlight... Would it move the air that is in the flashlight? I really have no idea Thanks!

Jared

When it gets power, it will turn around:laughing:

No seriously, that's a good question. Since the engine will produce heat too, it could be a negative effect for cooling.
Or it could be, that the hot air from the heatsink will quicker transfer the heat at the housing of the light, so it could be a positive thing.

But I don't know, it's only speculation.

Best regards

_____
Tom

Flow, whether gaseous or hydronic requires some form of pressure differential. If you could create this type of environment in your sealed light than yes, you would move air (very, very, very little volume though). But, in the thoughts of quenching a heat load, the heat that the moving air absorbs would would also need removed from the airstream as you would only be recirculating hot air. Kind of like I am doing now...:lolsign:

-DF

Hello there,

Assume you have the fan blowing on the inside part of a
heat sink where the outside of the heat sink is on the
outside of the flashlight. This could help because it will
be blowing the hotter inside air up against a cooler
heat sink, which will then conduct the heat to the outside
of the flashlight.

Without a heatsink like that, the only way to get the heat out
is through the flashlight walls. If the walls are highly conductive
the fan could help for the same reason as having the heatsink.
If the walls are not too conductive then it's going to be a
toss up depending on how much heat the motor adds to
the inside compartment...does the motor add more heat than
blowing the hot air against the inside walls removes?

Probably the only way to be sure is to try it.

Another thing that the fan could help with is to reduce temperature
gradients. When you have a real hot LED heatsink and the air
inside the light is cooler it would help to circulate the air.
Again, however, if the motor uses too much power it's certainly
going to add to the heat inside. For example, if you use a 1 watt
LED and the motor takes 0.5 watts of power, not only do you
loose a lot of run time but the motor most likely adds too much
heat to the inside to be helpful by circuilating the air in there.

Increase the drain on the battery, increase the size and weight of the flashlight, and increase the overall heat.
The light will have to be longer and if you want air flow, the diameter will have to be greater to allow the air to circulate. If there's not a lot of room for that more heat will be generated by frictional forces and you will need a bigger fan motor.
The problem of efficent air flow *WITHOUT* cutting holes in the flashlight is an exercise left to the reader.

I would not do a fan but a heat pipe to transfer heat down the barrel makes sense. Who will be the first to run heat pipes down a flashlight barrel? Running 4 K2's at 1,500 mA gets my vote!

BentHeadTX said:

I would not do a fan but a heat pipe to transfer heat down the barrel makes sense. Who will be the first to run heat pipes down a flashlight barrel? Running 4 K2's at 1,500 mA gets my vote!

Click to expand...

I am building one, with the machining help of my father. Using a 2D mag as a base I am running a heat pipe down the centre with 8xAA batteries around the pipe and 6xK2 in the head, copper heatsinks both ends connecting to aluminium fins. Both the head and tail will be finned.
I will post something when it is finished,but don't hold you breath, could easily be another 6 months at the rate we are going
matthew

jar3ds said:

What Would A Fan Do Inside a Sealed Flashlight?

Click to expand...

Nothing good!

Re: heat pipes. They take heat in on one side and spread it evenly across the whole pipe. So the idea is that at any given time, the whole pipe is the same temperature as the hot input end.

This is good because while aluminum conducts heat well, it doesn't do it nearly as well as that. If you look at a thermal image of a flashlight-size chunk of aluminum with a heat source on it, you'll see that the temp drops significantly as you move away from the heat source. For instance, it might be 10 degrees lower every centimeter. That is why useful heatsinks don't have super tall fins -- the effectiveness of the extra length drops significantly the more length you add. So you'll notice that most every solid heatsink will have fins no longer than a couple cm.

That's where heat pipes come in. If your application needs more heat sinking than it can get from that fixed area of short fins, you plug a heat pipe into the equation and suddenly you can transfer this heat to MORE short fins than before. That's the key. Heat pipes don't directly let you spit more heat into the air. They just let you shunt some heat to a place where you have more fins.

To illustrate, see the best performance CPU heatsink today: http://www.overclockers.com/articles1329/ It uses heat pipes to spread heat from one small source to a much larger area where it can be wicked away. This type of heatsink is heat and shoulders beyond the best traditional heatsinks from just a few years ago.

Fun bonus: note how there are no fins on the base plate. This particular manufacturer actually started building these heatpipe heatsinks with fins there. See the left part here: http://www.ap0calypse.com/scottrempel/si120/si12002.jpg But under testing you actually see that the extra fins either make ZERO difference, or so little difference that they aren't worth the extra penny of machining. Thus you see the newer generation of parts above and on the right of that last image. No fins on the base because it's not very hot -- the heatpipes THAT effective at removing heat from the base.

So here's the flashlight connection. A hot flashlight will basically have a really hot head, and then everything else (the body) is just a minor bonus. The dissipation performance of the head makes or breaks the system. I know from experience with my U2 on high that about an inch of head length gets really hot, and the rest significantly cooler.

So if you want to increase the cooling performance of a flashlight with heat pipes, you want to build heatpipes into the flashlight in such a way that it shunts heat from the heat to somewhere else with more dissipation performance.

My thoughts on a heatpipe design are as follows. Unless you plan to have a stack of fins on the tail, running a heatpipe to the tail won't do much of anything. And I don't really see a finned tail cap working out well. Better to just increase the fin area on the head and work through the heat pains.

So what would I do instead? I'd use the aluminum body and knurling to my advantage. I'd run a heatpipe or two from the head down inside of either side of the sufficently thick and well-knurled body.

A pretty massive undertaking for a custom light. And for questionable benefit -- LEDs are getting cooler and more powerful every year. In 10 years the thought of using heatpipes in a flashlight will be ridiculous. Even now it is overkill only necessary for use in massive LED arrays.

Conclusion: A fun idea, but probably impractical and unnecessary

mpf said:

I am building one, with the machining help of my father. Using a 2D mag as a base I am running a heat pipe down the centre with 8xAA batteries around the pipe and 6xK2 in the head, copper heatsinks both ends connecting to aluminium fins. Both the head and tail will be finned.
I will post something when it is finished,but don't hold you breath, could easily be another 6 months at the rate we are going
matthew

Click to expand...

Be sure to read and understand Lommy's post. You will be transferring heat along the length of the pipe to the batteries and only the small surface area at the end will be transferring heat out of the system.
Note also that some heat sinks are gravity dependant for much of their transfer ability.

Also be aware that a heat sink is essentially a storage device for heat and that a large surface area to actually get rid of the heat would be a better choice that a small surface area sink. Once the sink is "full" there are no advantages to it.

Mike Painter said:

...You will be transferring heat along the length of the pipe to the batteries and only the small surface area at the end will be transferring heat out of the system.
Note also that some heat sinks are gravity dependant for much of their transfer ability.

Also be aware that a heat sink is essentially a storage device for heat and that a large surface area to actually get rid of the heat would be a better choice that a small surface area sink. Once the sink is "full" there are no advantages to it.

Click to expand...

Thanks for that.
The heat pipe I am using is for PC use so should operate horizontally.
I have already added significant finning to the head and now want to transfer the heat down to the tail finning. I am aiming for 'real' dissipation not just a big heat 'sink'. Although given the battery capacity, the system may not reach steady state on a single set of batteries.
The copper is mainly for efficient transfer of the heat from the 6xK2 leds to the heat pipe and from the heat pipe to the tail finning. Since the contact area between the the heat pipe and the heat sink is fairly small, the copper is used to spread this out into a large contact area to transfer the heat to the aluminium fins.
The batteries will soak up some heat or add to it, depending on the load but the thermal contact between the batteries and the heat pipe is poor so I don't expect much heat transfer between these.

Loomy said:

Nothing good!
...
My thoughts on a heatpipe design are as follows. Unless you plan to have a stack of fins on the tail, running a heatpipe to the tail won't do much of anything. And I don't really see a finned tail cap working out well. Better to just increase the fin area on the head and work through the heat pains.
...

Click to expand...

Well I am planning a finned tail cap. On a 2D mag the finning should be behind your grip and out of the way. Although the new leds will run hotter, they loose output as the temperature goes up. Also the head on my current 20W led torch gets too hot to hold after 20min (no fins) so I am looking for more dissipation for the planned 30W k2 version. I know people suggest using your hand as a heat sink but I don't like that idea for 30Watts.

P.S. I am also developing a controller to handle this power and give three levels of light, low battery cutout, time out etc. That is going much better and is running my 20W torch now.